1. Technical Field
The inventive concept relates to a semiconductor integrated circuit, and more particularly, to a resistive memory device.
2. Related Art
Resistive memory devices use a resistive material of which a resistance is sharply changed based on an applied voltage to switch at least two different resistance states. There are typically a phase-change random access memory (PCRAM), a resistive random access memory (ReRAM), and a magnetoresistive random access memory (MRAM) as the resistive memory devices.
Among the resistive memory devices, the PCRAM determines data to be stored in the selected memory cell based on a crystalline state of a phase-change material. By heating the phase-change material, a phase of the phase-change material may be changed, and thus the resistance state may be controlled. The PCRAM has advantages of stability, unnecessity of an erase operation, endurance, support of access in byte units in addition to non-volatility and support of high-speed operation.
Recently, to obtain high capacity and high integration, the PCRAM is required to support a multi-level cell structure.
FIGS. 1A and 1B are cross-sectional views for explaining characteristics of a conventional PCRAM.
As illustrated in FIG. 1A, the conventional PCRAM includes a semiconductor substrate 1 in which a bottom structure including an access device (not shown) and a heating electrode (not shown) is formed, a phase-change material layer 5 formed to be in contact with the heating electrode within the semiconductor substrate 1, and an upper electrode 7 formed on the phase-change material 5. The phase-change material 5 is insulated from the adjacent phase-change material by an insulating layer 3.
The phase-change material 5 may be formed using a chalcogenide material, for example, germanium-antimony-tellurium (hereinafter, referred to as GST or Ge—Sb—Te). However, the GST material causes a phase-separation due to repeated crystallization and amorphization.
Therefore, as illustrated in FIG. 1B, as the number of use of the PCRAM is increased, antimony (Sb) within the GST material migrates toward a side of the heating electrode, and tellurium (Te) within the GST material migrates toward a side of the upper electrode 7.
FIGS. 2A and 2B are distribution diagrams for explaining the phase-separation of the GST material.
FIG. 2A shows the distribution of elements constituting a GST material in an initial state, and FIG. 2B shows the distribution of element constituting the GST material after the certain number of repeated write operations, for example, program operations, are performed.
As seen from FIG. 2A, tellurium (Te) and antimony (Sb) are uniformly distributed in a side of an anode, for example, an upper electrode and a side of a cathode, for example, a heating electrode. However, as operation cycles are repeated, tellurium (Te) migrates toward the anode and is concentrated in the anode side, whereas antimony (Sb) migrates toward the cathode and is concentrated in the cathode side.
Such phase-separation is caused when the elements constituting the GST material are pulled by an electric field. A resistance of the phase-change material in an amorphous state becomes low.
FIGS. 3A and 3B are characteristic diagrams illustrating current and voltage characteristic changes of a PCRAM due to the repeated write operations.
FIG. 3A illustrates a change of a resistance-current characteristic of the PCRAM due to the repeated write operations. When comparing a resistance-current characteristic curve A11 in an initial operation and a resistance-current characteristic curve B11 after the phase-separation is caused by repeated operation cycles, it may be seen that a resistance of a phase-change material, for example, a GST material, after the phase-separation is reduced. Reduction in the resistance of the phase-change material means increase in a reset current and thus an operation voltage of the PCRAM is increased.
FIG. 3B illustrates a change of a current-voltage characteristic of a PCRAM due to the repeated write operations. When comparing a current-voltage characteristic curve A12 in an initial operation and a current-voltage characteristic B12 after the phase-separation, it may be seen that a voltage is totally dropped when the same driving current is applied. That is, it may be seen that a threshold voltage is lowered after the phase-separation of a phase-change material, for example, a GST material, and thus malfunction of the PCRAM may be caused.
FIGS. 4A and 4B are characteristic diagrams illustrating reliability change of a PCRAM due to repeated write operations.
First, FIG. 4A illustrates endurance change of a PCRAM due to the repeated write operations. It may be seen that as the number of write operations is increased, a resistance of a phase-change material, for example, a GST material, is lowered from A13 to B13, and thus endurance is degraded.
FIG. 4B illustrates a change of a retention characteristic of a PCRAM due to the repeated write operations of the PCRAM. When comparing the retention characteristic curve A14 in an initial operation and the retention characteristic curve B14 after the phase-separation, it may be seen that the data retention characteristic is remarkably degraded.
As described above, the reliability of PCRAM is degraded and the lifespan of PCRAM is limited due to the phase-separation of the phase-change material, for example, a GST material, caused by the repeated operations.
To alleviate these concerns, refresh operation of the PCRAM may be considered. However, when the refresh operation, including, for example, an erase operation, is performed in a separate operation period, a time for the refresh operation may be additionally needed, and thus write latency may be increased.